The Global Positioning System (GPS) has been used to determine positions of mobile terminals. The GPS determines a position of a mobile terminal by receiving radio waves from a plurality of satellites. Therefore, it is difficult to receive the GPS service in underground malls and buildings where radio waves from the satellites are blocked.
In order to address the above difficulty, a visible light communication system including an illumination device has been conceived to detect positions of mobile terminals in underground malls and buildings (for example, Patent Literatures 1 and 2).
Patent Literature 1 discloses a visible light communication system applicable to an illumination device. FIG. 9 shows a configuration of the visible light communication system disclosed in Patent Literature 1.
The communication system 100 using visible light includes a transmitting device 110 and a receiving device 130. In the communication system 100, visible light is emitted from a light-emitting element 117 in the transmitting device 110, and then modulated according to a Quadrature Amplitude Modulation (QAM) signal generated from high-frequency carrier waves. After that, the resulting QAM-modulated signal is received by a light-receiving element 133 in the receiving device 130, and then demodulated. Here, the QAM-modulated signal is a signal of the visible light applied with the QAM modulation. The communication system 100 has a function of keeping a level of a received signal constant by using an Automatic Gain Control (AGC) 136. Furthermore, a communication system disclosed in Patent Literature 2 includes: a modulation unit that modulates sub-carrier waves; a light-blinking control unit that causes visible light to blink according to a modulated signal; and a ratio control unit that controls visible light to be emitted by changing a ratio of a light-emitting duration.
The visible light communication systems disclosed in Patent Literatures 1 and 2 can offer stable data communications using a modulated signal generated from high-frequency carrier waves. More specifically, in order to achieve stable data communications, Patent Literature 1 uses a QAM-modulated signal that is generated by multiplexing frequencies of carrier waves, and Patent Literature 2 controls visible light to blink according to a modulated signal using sub-carrier waves.
These communication systems can be used to determine a position of a mobile terminal (receiving device) even in an underground mall, if the mobile terminal receives visible light including ID information from an illumination device (transmitting device) provided in the underground mall.
Here, a communication distance between the illumination device and the mobile terminal changes according to a position of a user holding the mobile terminal. As the user walks, the communication distance changes with time. In the visible light communications, as a communication distance between a transmitting device and a receiving device is longer, a level of a received signal is drastically decreased. For example, a level of a received signal with a communication distance of 4 m is one sixty-fourth ( 1/64) of a level of a received signal with a communication distance of 0.5 m. In order to address such a significant change of a level of a received signal, Patent Literature 1 uses the AGC 136 to automatically adjust a level of an output signal to be constant, and then demodulates a visible light signal according to the output signal of the AGC circuit.